1. Field of the Invention
This invention is related to an optical system for use with short wavelength radiation in photolithography equipment.
2. Background of the Invention
Photolithography is a well known manufacturing process used to create devices upon substrates. The process typically involves exposing a patterned mask to collimated radiation, producing patterned radiation, which is passed through an optical reduction system. The reduced patterned radiation or mask image is projected onto a substrate coated with photoresist. Radiation exposure changes the properties of the photoresist allowing subsequent processing.
Photolithography machines or steppers, use two common methods of projecting a mask image onto a substrate, "step and repeat" and "step and scane". The step and repeat method sequentially exposes portions of a substrate to a mask image. The step and repeat optical system has a projection field which is large enough to project the entire mask image onto the substrate. After each image exposure, the substrate is repositioned and the process is repeated.
In contrast, the step and scan method in contrast scans a mask image onto a substrate through a slit. Referring to FIG. 1, a ring field lithography system 100 for use in the step and scan method is shown. A moving mask 101 is illuminated by a radiation beam 103, which reflects off of the mask 101 and is directed through a reduction ring field optical system 107. Within the optical system 107 the image is reversed and a slit shaped reduced image beam 109 is projected onto a moving substrate 111. The slit shaped reduced image beam 109 can only project a portion of the mask 101, thus the image beam 109 must scan the complete mask 101 onto the substrate 111. Because the mask 101 and substrate 111 move synchronously, a sharp image is scanned onto the substrate 111. Once the complete mask 101 is scanned onto the substrate, the mask 101 and substrate 111 are repositioned and the process is repeated. The dimensions of the slit are typically described by a ring field radius and a ring field width.
As manufacturing methods improve, the minimum resolution dimension which can be achieved with reduced pattern radiation decreases allowing more electronic components to be fabricated within a given area of a substrate. The number of devices that can be fabricated within an area of substrate is known as device density. A common measure of device density is the amount of memory that can be fabricated on a single DRAM chip. The relationship between resolution dimension and DRAM memory size is shown in Table 1. With existing technology 0.25 .mu.m resolution is possible.
TABLE 1 ______________________________________ Resolution Dimension DRAM Memory Size ______________________________________ 0.30 .mu.m 16-64 Megabytes 0.25 .mu.m 64-256 Megabytes 0.18 .mu.m 1 Gigabyte 0.13 .mu.m 4 Gigabyte 0.10 .mu.m 16 Gigabyte 0.07 .mu.m 64 Gigabyte ______________________________________
One well-known means of improving the resolution dimension and increasing device density is to use shorter exposure wavelength radiation during photolithography processes. The relationship between resolution dimension and radiation wavelength is described in the formula: R=(K.sub.1 .lambda.)/(NA), wherein R is the resolution dimension, K.sub.1 is a process dependent constant (typically 0.7), .lambda. is the wavelength of the radiation, and NA is the numerical aperture of the optical system projecting the mask image. Either reducing, or increasing the NA will improve the resolution of the system.
Improving the resolution by increasing the numerical aperture (NA) has several drawbacks. The most prevalent drawback is the concomitant loss in depth of focus with increased NA. The relationship between NA and depth of focus is described in the formula: DOF=(K.sub.2 .lambda.)/NA.sup.2, wherein DOF is depth of focus, and K.sub.2 is a process dependent constant (typically dose to unity). This simple relationship shows the inverse relationship between DOF and NA. For several reasons, including practical wafer flatness and scanning stage errors, a large depth of focus is on the order of .+-.1.0 micrometers is desirable.
Immediately, the shortcomings of resolution improvement via numerical aperture increase can be seen. As lithography technologies evolve toward shorter wavelengths, projection systems operate in different regions of wavelength-NA space. For EUV lithography at an operational wavelength of 13.4 nm, 0.1 .mu.m resolution can be achieved with a projection system that has a numerical aperture of 0.10. This low numerical aperture affords a depth of focus of .+-.1 .mu.m. In stark contrast, deep ultraviolet (DUV) lithography at 193 nm requires a projection system with a numerical aperture of 0.75 to achieve 0.18 .mu.m features (assuming K.sub.1 =0.7). At this NA, the depth of focus has been reduced to .+-.0.34 .mu.m. In addition, fabrication and assembly tolerances make it impractical to build optical systems with such as large NA.
As is known in the art, short .lambda. radiation (less than about 193 nm) is not compatible with many refractive lens materials due to the intrinsic bulk absorption. To reduce the radiation absorption within an optical system reflective elements may be used in place of refractive optical elements. State of the art DUV systems use catadioptric optical systems which comprise refractive lenses and mirrors. The mirrors provide the bulk of the optical power and the lenses are used as correctors to reduce the field dependent aberrations.
To produce devices with smaller resolution dimensions and higher device density than is possible with DUV systems, optical systems compatible with even shorter wavelength radiation are required. Extreme ultraviolet (EUV) radiation (.lambda. less than about 15 nm) cannot be focused refractively. However, EUV radiation can be focused reflectively using optical elements with multilayer coatings.
Early EUV lithographic projection optical systems focused on the development of optical systems that project two dimensional image formats. One example of a step and repeat optical system is disclosed in U.S. Pat. No. 5,063,586. The '586 patent discloses coaxial and tilted/decentered configurations with aspheric mirrors which project approximately a 10 mm.times.10 mm image field. The '586 patent system achieves an acceptable resolution of approximately 0.25 .mu.m across the field, but suffers from unacceptably high distortion, on the order of 0.7 .mu.m. The '586 patent optical system is impractical because the mask would have to pre-distorted in order to compensate for the distortion.
More advanced step and scan optical systems have been developed due to the unacceptable distortion of the large image fields of step and repeat optical systems. Step and scan systems have less distortion than step and repeat systems because distortion can be corrected over the narrow slit width in the direction of scan. Step and scan optical systems typically utilize ring fields. Referring to FIG. 2, in a step and scan optical system an image is projected by the inventive optical system onto the wafer through an arcuate ring field slit 201 which is geometrically described by a ring field radius 203, a ring field width 205 and a length 207. Ring field coverage is unlimited in azimuth.
One example of a step and scan optical system is disclosed in U.S. Pat. No. 5,315,629. Although the '629 patent optical system has low distortion, the ring field slit width 205 is only 0.5 mm at the wafer. High chief ray angles make it difficult to widen the ring field width 205. The 0.5 mm width 205 of the '629 patent limits the speed at which the wafer can be scanned, restricting throughput. The radius 203 of the '629 patent optical system ring field is 31.5 mm which does not cover a wide swath in the cross scan dimension, further reducing wafer throughput.
Another disadvantage of systems similar to the '629 patent optical system is that it may require the use of graded multilayer reflective coating optics as opposed to simpler uniform thickness multilayer coating optics. Uniform thickness multilayer coatings are generally not suitable for high incidence angles and a wide range of incidence angles. FIG. 3 illustrates the non-uniform reflectivity resulting from high and wide ranges of incidence angles against a uniform multilayer optical element. Beams 301 and 303 have incident angles of 10.degree. and 15.degree., which correspond to reflectivities of 69% and 40%, respectively. The reflected intensity of reflected beam 309 is greater than the intensity of reflected beam 311 because the incidence angle of beam 301 is in a higher reflectivity region than the incidence angle of beam 303. This difference in the resulting reflectivity creates a projected image with non-uniform intensity.
Referring again to FIG. 3, if a graded reflective coating is properly applied to optical element 305, the reflectivity at the incidence point of beam 301 is reduced so that the reflected beam 309 has an intensity equal to that of beam 311. Although graded reflective optics can address the intensity uniformity problem, graded reflective optics are nonetheless undesirable because they are difficult to fabricate and test.
Another example of a step and scan optical system is U.S. Pat. No. 5,353,322. The '322 patent discloses 3-mirror and 4-mirror optical systems. An extra mirror in the 4-mirror systems solves the optical system wafer/mask clearance problem but does not add any reflective power or provide any optical aberration correction. Another drawback of the '322 optical system is that its aperture stop is inaccessible. Because the aperture stop is inaccessible, the imaging bundle will change as the projected image crosses the ring field width.
There are a number of other prior art optical systems compatible with EUV wavelength radiation and use reflective optics. These prior art EUV optical systems typically use simple reflective optics which have spherical convex or spherical concave surfaces. The surface of a spherical reflective element is defined by a constant radius of curvature across the surface of the optic. A drawback of all spherical systems is that they can distort projected images by introducing unwanted aberrations (i.e. spherical, coma, astigmatism, Petzval curvature and distortion). These aberrations can be at least partially corrected or even eliminated by using aspheric mirrors.
In view of the foregoing, there is a need for an optimized optical system which is compatible with short wavelength radiation, has high radiation throughput, uses only uniform thickness multilayer reflective coating optics, uses only low aspheric reflective optics and has an accessible aperture stop.